1. Field of the Invention
This invention relates to air conditioning systems and more particularly to air conditioning systems that control humidity levels in the air. It is also applicable to removing or reducing the moisture content from other gases.
2. Description of Related Art
Air-conditioning, the cooling of the air of indoor spaces, has been known for decades. Residential or home air-conditioning is typically controlled by a thermostat that solely measures the dry bulb temperature or sensible heat of the air being conditioned.
However, one of the main objectives of air-conditioning is to reduce the moisture content or relative humidity of the air as well. Depending on the temperature and humidity, controlling an air-conditioning system based on dry bulb temperature alone may produce uncomfortable conditions and use energy inefficiently. A system that does not effectively dehumidify the air may leave the conditioned air cold but damp; for example, it is more comfortable to maintain a room at 75.degree. F. at low humidity conditions than it is to cool a room to 68.degree. F. but allow humidity levels to remain high.
One proposed solution has been to add a relative humidity sensor to the typical thermostat of an air-conditioning system and then control the air-conditioner to hold relative humidity within a predetermined range. There are several drawbacks to such a system. For one, humidity sensors that directly measure relative humidity are generally inaccurate and slow to respond to changes in ambient conditions. A problem may also result with such a system in that the relative humidity of the enclosed air will rise as it is cooled provided one remains above the dew point. Relative humidity is a function of both the amount of water vapor per volume of air and its dry-bulb temperature, specifically the ratio of the partial pressure of the water vapor in the air to the vapor pressure of saturated steam at that pressure. Since the vapor pressure of saturated steam drops rapidly with temperature, a small amount of water vapor at a lower temperature can result in 100% relative humidity. It is possible to have a situation where the humidity control calls for further dehumidification, the temperature drops, and the relative humidity rises.
Air-conditioning systems are sized to reduce both the sensible and latent heat at the design point. This is a point where the known maximum load may occur only 5-10% of the time during the cooling season based on local climatological weather data and utilization of the space. During periods when the sensible heat load is less than the design point and the relative humidity is high, a dry bulb thermostat will only need to run the air-conditioner for a short period of time before the space reaches the desired dry bulb set point and the thermostat is satisfied. This short run time does not allow the air-conditioner sufficient time to remove the required amount of moisture from the air and therefore the air will be cool and damp, an uncomfortable condition.
FIG. 1 shows the typical airflow in an air-conditioning system. The incoming air stream 1 is pulled through a cooling coil 2 by a fan 3 and leaves as cooler air 4. Moisture will be removed from the incoming air stream 1 if the coil temperature 2 is at or below the point of condensation (dew point) of the incoming air 1 and will leave as a dehumidified air 4. A thermostat 5 measures the dry bulb temperature by means of a temperature sensor 7 in the air of the conditioned space. Once the dry bulb air temperature is cooled to the desired point, the thermostat turns off the compressor or cooler 6 and stops the process of removing sensible heat and moisture. The occupant of the conditioned space may have an option depending on the model of thermostat as to whether or not the thermostat 5 will stop the fan 3 from moving air or continue to run the fan and circulate air in the conditioned space during the period when no cooling is required. By controlling the cooler 6 based on the dry bulb temperature sensor 7 connected to the thermostat 5, there may be periods when the conditioned space is uncomfortable due to high levels of humidity for the given set point of the thermostat 5.